The present invention is directed to chillers providing a chilled fluid such as water for use in air conditioning, refrigeration and the like. More particularly, the present invention is directed to the control of a variable flow chilled water system and represents an improvement in chiller capacity control.
A chiller is a device providing a chilled fluid. For many years it was considered necessary that chillers be constant flow devices. Articles such as "Why Must Chillers be Constant Flow Devices?", the Trane Engineering Newsletter, Volume 9, Number 1, January-February 1980 cite reliable heat transfer from a temperature control and instability in the control system as reasons supporting the premise that chillers must be constant flow devices.
In recent years this has changed and variable flow chilled water systems have begun to be implemented. An article "Don't Ignore Variable Flow" by James P. Waltz was published in the July 1997 issue of Contracting Business (pages 133-144) discussing basic variable flow chilled water systems. On the final page of the article, the author recognizes the ongoing problems in controlling variable flow chilled water systems. Problems are cited regarding minimum flow control, incorrect speed control, and insufficient cooling or flow capacity.
Many of these problems result from a chiller system which cannot compensate for simultaneous changes in the capacity of one or more chillers and in the flow rate of the chilled water being provided to the air handlers.